Yep if booking through a third party package holiday (such as Virgin Holidays) they do push the flex ticket. Even booking direct with Universal they push towards a three park with WetnWild or the flex ticket.
I think you can easily fill a seven night trip with Universal, IoA, WetnWild, Florida Mall / Premium Outlets, SeaWorld. Add in Aquatica & Busch Gardens plus a second day at the Universal parks and you can fill a 10 night trip without even going to Disney.
There may also be people who stay at Universal Hotel and then drive to Disney for the day now. I used to do the opposite (stay Disney, drive to Universal) but Harry Potter has tempted me to stay Universal and spend less time at Disney.
One side note on the Flexticket, I found it cheaper on our last trip to buy a 2-park universal ticket and then a 2 park Busch/Seaworld ticket with Aquatica free. We weren't planning on doing wet'n'wild as already doing both Disney waterparks and Aquatica so this was a better deal. All the UK options still give 14 days admissions.